Computational Investigation of Coaxial Rotor Interactional Aerodynamics in Steady Forward Flight

This paper examines the aerodynamic environment of a high speed coaxial compound helicopter in steady level forward flight using CFD-CSD coupling. Numerical simulations have been performed to study trim configurations of an eight-bladed coaxial compound configuration (four blades per rotor) using comprehensive analysis coupled to a vortex wake model. The model is validated using public-domain information and flight test data obtained for the Sikorsky X2 Technology Demonstrator. Rotor-rotor interference follows the expected momentum theory type model, with the lower rotor experiencing larger induced inflow. Thrust sharing between rotors remains nominally constant, with at most ±5% bias over the speed range studied. CFD analysis for the rotors is then introduced at select forward flight speeds to identify key aerodynamic modeling refinements. In particular, impulsive normal force due to rotor blade crossings are not captured accurately by the vortex wake model. The impulsive 8/rev pitching moment is completely missed by the reduced order model. While significant, this aerodynamic pitching moment does not excite a noticeable torsion response from the stiff rotor considered. For future designs with lower stiffness blades, these impulsive airloads may induce additional vibratory hub loads and drive the design of the rotor pitch control system.

[1]  W. Johnson,et al.  CAMRAD - A COMPREHENSIVE ANALYTICAL MODEL OF ROTORCRAFT AERODYNAMICS AND DYNAMICS , 1994 .

[2]  Ananth Sridharan,et al.  Simulation modeling of flight dynamics, control and trajectory optimization of rotorcraft towing submerged loads , 2014 .

[3]  J. Anderson,et al.  Fundamentals of Aerodynamics , 1984 .

[4]  Mark Potsdam,et al.  Rotor Airloads Prediction Using Loose Aerodynamic/Structural Coupling , 2004 .

[5]  A.M. Wissink,et al.  Large Scale Parallel Structured AMR Calculations Using the SAMRAI Framework , 2001, ACM/IEEE SC 2001 Conference (SC'01).

[6]  Robert,et al.  Dynamics Design Characteristics of the Sikorsky X2 TechnologyTM Demonstrator Aircraft , 2008 .

[7]  Scott R. Kohn,et al.  Managing application complexity in the SAMRAI object‐oriented framework , 2002, Concurr. Comput. Pract. Exp..

[8]  James D. Baeder,et al.  High-Resolution Computational Investigation of Trimmed Coaxial Rotor Aerodynamics in Hover , 2009 .

[9]  Baeder,et al.  Validation of UH-60A Rotor Blade Aerodynamic Characteristics Using CFD , 2003 .

[10]  David A. Peters,et al.  Comparison of Measured Induced Velocities with Results from a Closed-Form Finite State Wake Model in Forward Flight , 1989 .

[11]  Hyeonsoo Yeo,et al.  Validation of Rotorcraft Comprehensive Analysis Performance Predictions for Coaxial Rotors in Hover , 2017 .

[12]  Ye Zhengyin,et al.  Numerical Simulation of Unsteady Flow Around Forward Flight Helicopter with Coaxial Rotors , 2011 .

[13]  Farhan Gandhi,et al.  A Physics-Based Approach to Trim Optimization of Coaxial Helicopters in High-Speed Flight , 2015 .

[14]  Richard E. Brown,et al.  Interactional aerodynamics and acoustics of a hingeless coaxial helicopter with an auxiliary propeller in forward flight , 2008 .

[15]  Andrew J. Ruddell.  Advancing Blade Concept (ABC Tm) Development , 1977 .

[16]  Inderjit Chopra,et al.  Aeromechanics for a High Advance Ratio Coaxial Helicopter , 2015 .

[17]  Datta,et al.  Analysis Refinements for Prediction of Rotor Vibratory Loads in High-Speed Forward Flight , 2004 .

[18]  R. K. Burgess.  Development of the ABC Rotor , 1971 .

[19]  J. Gordon Leishman,et al.  Predictions of Rotor and Rotor/Airframe Configurational Effects on Brownout Dust Clouds , 2014 .

[20]  M. C. Cheney,et al.  The ABC helicopter , 1969 .

[21]  Mathieu Amiraux,et al.  NUMERICAL SIMULATION AND VALIDATION OF HELICOPTER BLADE-VORTEX INTERACTION USING COUPLED CFD/CSD AND THREE LEVELS OF AERODYNAMIC MODELING , 2014 .